Interferometric techniques are commonly used to obtain information about a test object, such as to measure the profile of a surface of the test object. To do so, an interferometer combines test light reflected from the surface of interest with reference light reflected from a reference surface to produce an interferogram (also referred to as an interference pattern). Fringes in the interferogram are indicative of spatial variations between the surface of interest and the reference surface. In many implementations, an interferometer acquires a series of interferograms using a multi-element detector, each interferogram being subject to a slightly varied phase difference between the test light and the reference light. For each detector element, the series of acquired intensity values provides an interferometry signal from which information about the test object can be determined.
A variety of interferometric techniques have been successfully used to characterize a test object. These techniques include low coherence scanning techniques and phase-shifting interferometry (PSI) techniques.
With PSI, the optical interference pattern is recorded for each of multiple phase-shifts between the test light and reference light to produce a series of spatial optical interference patterns that span, for example, at least a half cycle of optical interference (e.g., from constructive, to destructive interference). The sequence of optical interference patterns define a series of intensity values for each spatial location of the pattern, wherein each series of intensity values typically has a sinusoidal dependence on the phase-shifts with a phase-offset equal to the phase difference between the combined test light and reference light for that spatial location. Using numerical techniques, the phase-offset for each spatial location is extracted from the dependence of the intensity values to provide a profile of the test surface relative the reference surface. Such numerical techniques are generally referred to as phase-shifting algorithms.
The phase-shifts in PSI can be produced by changing the optical path length from the measurement surface to the interferometer relative to the optical path length from the reference surface to the interferometer. For example, the reference surface can be moved relative to the measurement surface. Alternatively, the phase-shifts can be introduced for a constant, non-zero optical path difference by changing the wavelength of the measurement and reference light. The latter application is known as wavelength tuning PSI and is described, e.g., in U.S. Pat. No. 4,594,003 to G. E. Sommargren.
Low coherence scanning interferometry, on the other hand, scans the optical path length difference (OPD) between the test light and reference light over a range comparable to, or larger than, the coherence length of the interfering test and reference light, thereby producing a scanning interferometry signal for each detector element used to detect the interferograms. The coherence length of the light is relatively short compared to the coherence length of light commonly used for PSI and relative to the range of OPD's scanned in a measurement. A low coherence length can be produced, for example, by using a white-light source, which is referred to as scanning white light interferometry (SWLI). A typical SWLI signal is a few fringes localized near the zero OPD position. The signal is typically characterized by a sinusoidal carrier modulation (the “fringes”) with bell-shaped fringe-contrast envelope. The conventional idea underlying low coherence interferometry metrology is to make use of the localization of the fringes to measure surface profiles.
Low coherence interferometry processing techniques include two principle trends. The first approach is to locate the peak or center of the envelope, assuming that this position corresponds to the zero OPD of a two-beam interferometer for which one beam reflects from the object surface. The second approach is to transform the SWLI signal into the frequency domain and calculate the rate of change of phase with wavelength, assuming that an essentially linear slope is directly proportional to object position. See, for example, U.S. Pat. No. 5,398,113 to Peter de Groot. This latter approach is referred to as Frequency Domain Analysis (FDA).
Low coherence scanning interferometry can be used to measure surface topography and/or other characteristics of objects having complex surface structures, such as thin film(s), discrete structures of dissimilar materials, or discrete structures that are underresolved by the optical resolution of an interference microscope. Such measurements are relevant to the characterization of flat panel display components, semiconductor wafer metrology, and in-situ thin film and dissimilar materials analysis. See, e.g., U.S. Patent Publication No. US-2004-0189999-A1 by Peter de Groot et al. entitled “PROFILING COMPLEX SURFACE STRUCTURES USING SCANNING INTERFEROMETRY” and published on Sep. 30, 2004, the contents of which are incorporated herein by reference, and U.S. Patent Publication No. US-2004-0085544-A1 by Peter de Groot entitled “INTERFEROMETRY METHOD FOR ELLIPSOMETRY, REFLECTOMETRY, AND SCATTEROMETRY MEASUREMENTS, INCLUDING CHARACTERIZATION OF THIN FILM STRUCTURES” and published on May 6, 2004, the contents of which are incorporated herein by reference.
As explained above, interferometric techniques are used to gain information about an object by analyzing interferograms created by interfering test light from the object and reference light from a reference object. Because the interferogram depends on the relative position between the test object and the reference object, the precision with which the relative position is known influences the quality of the analysis of data acquired using the interferometric technique and, therefore, the information gained about the test object. Accordingly, it can be advantageous to monitor the relative position. In general, the relative position between the test object and the reference object can be monitored in a number of ways. For example, one can monitor the motion of the test object or reference object relative to other components in the interferometer based on a position output signal of a translation stage that is used to vary the position of the test or reference objects (i.e., that varies the OPD), as described in, for example, U.S. Pat. No. 6,822,745 by Peter de Groot et al, entitled “OPTICAL SYSTEMS FOR MEASURING FORM AND GEOMETRIC DIMENSIONS OF PRECISION ENGINEERED PARTS.” In some embodiments, an optical calibration of the scan motion of the translation stage can be used. See, e.g., U.S. Pat. No. 7,446,882 by Xavier Colonna de Lega et al. entitled “INTERFEROMETER FOR DETERMINING CHARACTERISTICS OF AN OBJECT SURFACE” and published Nov. 4, 2008, the contents of which are incorporated herein by reference.